This invention relates to an amplifier, more particularly to an analog audio amplifier having both excellent linearity and high efficiency, which is combined with digital amplifier.
A conventional analog audio amplifiers such as class A, class B or class AB, based on the linear circuit technology, have very excellent distortion characteristics, but show considerably poor efficiency and bulky volume. On the other hand, the class D type digital amplifier, which adopt a pulse width modulation(PWM) switching, has high efficiency and small volume but relatively bad fidelity.
To date with the event of so-called Green Round, particularly in electronic products including audio amplifiers, it is necessary that the higher efficiency technology should be developed in order to reduce the loss of energy.
Conventionally, in the audio amplifier, the most important factor on the product""s design was the linearity rather than the efficiency under the circumstances which have no particular interest in power loss or due to the technical difficulty. As a result, almost all the amplifiers for audio amplifier application till now are based on the analog linear circuit having an excellent distortion property.
However, although audio amplifiers which adopt class A, class B or class AB circuit have excellent linear characteristics as discussed above, it inevitably causes a large quantity of power loss in the high power amplifier. Therefore, in the typical analog audio amplifiers, since the most of output except for the energy converted to the audio sound is converted to a thermal energy and dissipated, this results in the temperature increase of the power transistor groups composed of bipolar transistors, field effect transistors, etc., requiring massive heat sink made of metals such as aluminum, copper, etc. or noisy fan for cooling, and thus undesirably increasing size and weight of the power amplifier system. As is generally known, since class A amplifier always operates in the active region, it simultaneously has merit and demerit of the most excellent distortion characteristics and the worst efficiency.
Thus, it would be desirable to provide an amplifier having both excellent linearity and high efficiency, which are difficult to be compatible. To solve this problem, class B type amplifiers composed of so-called push-pull circuit has been studied so far. The push-pull circuit includes a pair of transistors connected to each other in the emitter-follower fashion, which is very useful for energy saving. A high efficiency up to maximum 78.5% can be theoretically obtained, but undesirable crossover distortion problem between two transistors in the small signal level still persists. Additionally, in the class B type amplifiers, although the crossover distortion in the small signal level can be somewhat improved by applying a proper negative feedback, it is impossible to satisfactorily eliminate total harmonic distortion(i.e., THD) under the application of high voltage and flow of large amount of current. The reason why is that the two transistors constituting the class B type amplifiers are alternatively turned-on and turned-off and therefore allow easy switching into the turn-on and turn-off in the small signal level, but it is difficult to perform rapidly the switching operation owing to the charge storage effect in transistors when a large amount of current flows, resulting in degradation of the total harmonic distortion.
On the other hand, the class AB type amplifier, which has its load curve between class A and class B, allows small amount of current to flow even when no signal is applied. This amount of current is smaller than that of the class A type amplifier but remarkably larger than that of the class B type amplifier. In the class AB type amplifier, as the bias current increases, linear characterics thereof approaches to the class A, whereas as the bias current decreases, it approaches to the class B.
A simple analog audio amplifier to which negative feedback is applied will be described with reference to FIG. 1.
As well known, in class A, B or AB amplifier, about 21.5% to about 75% of supplied energy is dissipated as a thermal energy, and thus it is necessary for a large aluminum heat sink or noisy fan for cooling, mounted within the enclosure in which various devices are mounted. Therefore, such amplifiers have drawbacks of not only considerably poor efficiency and bulky volume, which are peculiar to typical class A, B or AB amplifier, but also incurring noise when driving the cooling fan. In particular, when such amplifiers are operated in the closed space such as vehicle, the characteristics of various devices including power transistors can be deteriorated due to poor radiation of heat within the enclosure, resulting in shortening of the life cycle in audio equipment.
Accordingly, an audio amplifier having both of a high fidelity and a high efficiency is strongly demanded.
This requirements can be solved by the present invention which provides the analog amplifier mixed with class D type digital amplifier having high efficiency and small volume.
The class D type digital amplifier adopts the pulse width modulation(PWM) switching scheme which performs amplifying function by switching operation other than linear operation. It is generally known that the class D type digital amplifier has a high efficiency and small volume but relatively bad fidelity. In the class D type digital amplifier, the gate signals of power transistors, used as control pulses, are generated by comparing the carrier signal of saw tooth wave with the control reference signal of error signal of audio signal.
The nonlinearity due to distortion inevitably caused in class D type digital amplifier should be corrected through precise negative feedback technology to meet the acoustic fidelity required for audio equipment.
The operating principle of class D type digital amplifier is identical with that of switching regulator or pulse width modulation(PWM) converter, except that class D type digital amplifier for audio equipment has wider bandwidth ranging about 20 Hz to 20 kHz, which covers the audible frequency band, compared with the switching regulator or PWM converter.
The class D type amplifier is generally provided with high power field effect transistors(FETs) as power switching device, and is possible to obtain the efficiency of 100% in theory, but, in fact, of about 90% due to thermal loss generated in proportion to the switching frequency and the power loss in various control circuits.
Since such class D type amplifier is very poor in distortion characteristics, to ensure the sound quality needed in acoustic equipments, it is necessary to design the precise feedback circuit. Generally, in acoustic equipment provided with class D type amplifier, the digital circuit block which performs amplifying operation by utilizing PWM type switching is mixed with the analog circuit block, bring about a large amount of switching noise in the circuit. Thus, it is difficult to design the negative feedback circuit with sufficient stability. Additionally, in case that the negative feedback circuit is erroneously designed, it may cause undesirable oscillation, often resulting in critical damage to the circuit block.
As one example of prior art for removal such disadvantages in class D type amplifier, there has been known xe2x80x9cTriple loop negative feedback audio amplifierxe2x80x9d (Korean patent application No.96-37905), applied by the present inventors. This triple loop negative feedback audio amplifier show very low distortion characteristics of about 0.1%, compared with the conventional class D amplifier, while maintaining the high efficiency and the high power.
Unfortunately, audio amplifiers which have been developed till now are constructed to have only one characteristic of a high fidelity and a high efficiency, depending on their using purpose.
Accordingly, there exists a need for an audio amplifier simultaneously having excellent distortion characteristics and high efficiency.
It is the general object of this invention to provide an analog amplifier simultaneously having a high fidelity and a high efficiency with aid of digital amplifier.
In order to achieve the above object, the analog-digital combined amplifier according to the present invention comprises a class A, class B or class AB type analog amplifier serving as the independent voltage source, and a class D type digital amplifier serving as a dependent current source thereof, which are connected to each other.
According to one embodiment of the present invention, the analog-digital combined amplifier further comprises a sensing circuit for sensing a small amount of current output from the analog amplifier and generating a sense voltage to allow the digital amplifier to supply a large amount of current.
According to another embodiment of the present invention, said analog amplifier comprises a basic amplifier; a base driver for controlling the operation of transistors; a pair of transistors constituting the push-pull stage; and a feedback means composed of passive elements for determining the closed-loop gain of a whole system.
According to still another embodiment of the present invention, said digital amplifier comprises a comparator; a plurality of resistors for determining the hysteresis voltage of said comparator and a pull-up resistor; a gate driver; a pair of high power switches constituting output stage; and an inductor.
According to still another embodiment of the present invention, said inductor has a value which is in range of about 10 xcexcH to about 500 xcexcH.
According to still another embodiment of the present invention, said sensing block comprises a resistor for sensing the current supplied from the analog amplifier or absorbed thereto and converting it into the voltage component; and an amplifier for amplifying the voltage sensed by said resistor.
According to still another embodiment of the present invention, said sensing resistor has a value which is in range of about 0.01xcexa9 to about 10xcexa9.